So sorry about Bella - do hope you find her safe and well.
I am not defending travellers, but ....we have some rogues round here who are not travellers, and are quite capable of taking hens, goats etc. so its not always travellers. Having said that, no one should think they have a right to help themselves to peoples much loved animals.
Have you a local livestock market? May be worth putting up a poster there, as obviously animal minded people will visit a market, and also mention it in the market office, so if a pygmy goat is brought in to sell, they can alert you. Just a thought, as I know they do it when horses go missing.
Also, contact animal rescue centres in your area, just in case someone has taken Bella, then realised after a day or two that you are looking for her, and dumped her in a field or something, and she has ended up at an animal centre.
A notice on the board in your local vets is also another good idea .....may jog peoples memories if they suddenly notice a new goat in their neighbourhood.